Design Considerations

Windows are a complex and interesting element in residential design. New window products and technologies have changed the performance of windows in a radical way. Issues such as climate, orientation, shading, and window area all effect the energy performance, but human factor issues such as access to fresh air, daylight, and natural views impact the comfort of a home.

Measuring Performance

Heat flows through a window assembly in three ways: conduction, convection and radiation. When these basic mechanisms of heat transfer are applied to the performance of windows, they interact in complex ways. Three energy performance characteristics of windows are used to portray how energy is transferred and are the basis for how energy performance is quantified.

Insulating value. When there is a temperature difference between inside and outside, heat is lost or gained through the window frame and glazing by the combined effects of conduction, convection, and radiation. This is indicated in terms of the U-factor of a window assembly.

Heat gain from solar radiation. Regardless of outside temperature, heat can be gained through windows by direct or indirect solar radiation. The ability to control this heat gain through windows is measured in terms of the Solar Heat Gain Coefficient (SHGC) of the window.

Infiltration. Heat loss and gain also occur by Air Leakage through cracks in the window assembly. This effect is measured in terms of the amount of air (cubic feet or cubic meters per minute) that passes through a unit area of window (square foot or square meter) under given pressure conditions. In reality, infiltration varies slightly with wind-driven and temperature-driven pressure changes.

Visible Transmittance is the amount of light in the visible portion of the spectrum that passes through a glazing material. This property does not directly affect heating and cooling loads in a building, but it is an important factor in evaluating energy-efficient windows.

Both the National Fenestration Rating Council (NFRC) and the American Architectural Manufacturers Association (AAMA) have developed systems for rating the condensation resistance of fenestration products. AAMA gives a dimensionless rating, the Condensation Resistance Factor (CRF), ranging from 30 to 80 and is based on a physical test. NFRC determines Condensation Resistance (CR) ranging from 1 to 100 and is based on computer simulation. There is no correlation between CRF and CR.